1. Field of the Invention
The present invention relates to an apparatus which controls the intensity of a laser beam emitted from a laser unit, and more particularly to a beam intensity-controlling apparatus adapted for use with a semiconductor laser unit incorporated in an image-forming apparatus, such as a laser printer or a digital copying machine.
2. Description of the Related Art
In recent years, an electrophotographic laser printer has been developed, which scans a photosensitive surface with the laser beam emitted from a laser unit for the purpose of exposure and which prints images and/or characters by utilization of an electrophotographic process. In this type of laser printer, the laser unit is constituted by a semiconductor laser unit. In general, the beam intensity of the laser beam emitted from the semiconductor laser unit has to be stabilized by a control device since the beam intensity is likely to vary in accordance with temperature.
There are various types of known control systems which can control the intensity of the laser beam emitted from the semiconductor laser unit. Of the known control systems, the control system which employs a general-purpose microcomputer or microprocessor incorporating an A/D converter is considered a low-cost type. In this type of control system, the semiconductor laser is energized to emit a laser beam and a detector detects the beam intensity of a laser beam during &he interval between the image-recording operation performed with respect to a given page and the image-recording operation performed with respect to the succeeding page, and the detected beam intensity is supplied to the A/D converter, for reading. The beam intensity read by the A/D converter is compared with a reference signal by the microcomputer and its peripheral circuits, and the current used for the driving of the semiconductor laser unit is controlled on the basis of the comparison. In this manner, the intensity of the laser beam emitted from the semiconductor laser unit is stabilized.
According to the conventional control system mentioned above, the intensity of the laser beam emitted from the semiconductor laser unit is stabilized, by causing the semiconductor laser unit to emit a laser beam in the interval which is between the image-recording operations performed with respect to two successive pages, i.e., in the interval in which the image-recording operation is not performed. If this control system is used in a certain type of image-forming apparatus (e.g., a laser printer or a digital copying machine to which a reversal development system is applied), the laser beam emitted in the interval between the image-recording operations may fall on a recording region. If this occurs, the image on the recording region is developed with toner, so that the toner is used in vain.
According to another type of control system which may solve the above problem, the intensity of a laser beam emitted from the semiconductor laser unit is stabilized, with the semiconductor laser unit being energized in the region other than the recording region. According to this control system, however, the time during which the semiconductor laser unit is energized is comparatively short, and the A/D converter incorporated in the general-purpose microcomputer requires a long time for reliably reading the detected beam intensity of the semiconductor laser unit. This being so, it is necessary to employ a high-speed A/D converter, i.e., a costly A/D converter. It is also necessary to employ a high-speed microcomputer since high-speed processing is required from the reading of the detected beam intensity to the determination of the driving current supplied to the semiconductor laser unit.
The conventional control apparatus mentioned above operates as follows. Upon receipt of a print command for commanding the start of printing, the conventional control apparatus first initializes the value of the current used for driving the semiconductor laser unit, and then controls the semiconductor laser unit such that its laser beam intensity attains a target beam intensity. Therefore, a certain time is required before the laser beam attains the target beam intensity.
In the case of an image forming apparatus, such as a laser printer or a digital copying machine, it is desirable that the time required from the input of the print- or copy-starting command to the execution of the first printing operation (or the production of the first copy) be short as possible. An image forming apparatus capable of promptly executing the first printing operation (or the projection of the first copy) is considered a high-performance apparatus. If the time required for the laser beam to attain the target beam intensity is long, the start of the copying or printing operation is delayed, so that the user has to wait for a long time before the first copy is produced.
In the case of a high-speed laser printer or digital copying machine, the time needed before the start of the first printing or copying operation is short. The higher the operation speed of the laser printer or digital copying machine is, the shorter will be the time required before the start of the first printing or copying operation. If, however, the time needed for attainment of the target beam intensity is long, it is not possible to make the most of the high-speed operation of the laser printer or digital copying machine.
According to the conventional control apparatus, when the laser beam emitted from the semiconductor laser unit is directed to the region other than the recording region, the semiconductor laser unit is energized, and the beam intensity of the laser beam is stabilized. The beam emission timing of the semiconductor laser unit is determined in accordance with a beam detection signal which is output by a beam detector when the laser beam reaches the region other than the recording region. In general, the beam detector is an optical detector. This detector cannot detect a laser beam incident thereon until the beam intensity of the semiconductor laser unit attains a given level which is high enough for the detector to sense. Therefore, in order to control the beam intensity of the semiconductor laser unit in the region other than the recording region, the driving current of the semiconductor laser unit has to be determined such that laser beam intensity can attain the given level, first of all.
The initial driving current has to be determined in consideration of the threshold current value and characteristic variation (e.g., in a variation in slope efficiency). Unless these factors are considered, it may happen that the laser beam emitted from the semiconductor laser unit will fail to attain the given beam intensity, resulting in failure in beam detection, even if the same value as used last time is set for the driving current. Conversely, it may happen that the laser beam will greatly exceed the given beam intensity, resulting in damage or adverse effects to the semiconductor laser unit.